UE4 flipping my UVs on import?

I am using the UV channel to store vertex data in the range of about -1.5 to 6.5 on U and -0.3 to 4 on V. When I bring the geometry into unreal and set up the material I get unexpected results for the V channel only, U is fine.

I can fix it by selecting “Un Mirror V” in the TexCoord node.

I would just like to know what is happening so I can be sure I’m applying the right fix. Unreal is doing something to the UVs on import for this error to occur but I don’t understand why it is happening now as I have imported many models with regular texture UVs and never had to use this option.

Could be because the data is over the 0-1 range. Though I can’t say I fully understand it.
in fact, could you explain what/how you are achieving by having extra data attached outside the 0 to 1 range? It’s something I should probably learn for my edification…

It’s just using the UV to store data like vertex colour would, then with a texcoord node and component mask you can extract that data. I don’t know if it’s more or less efficient than vertex colour.